Which of the following is a type of optical disc that users can read from but not write to or erase?

What is an optical disk?

An optical disk is an electronic data storage medium that can be written to and read from using a low-powered laser beam. Most of today's optical disks are available in three formats: compact disks (CDs), digital versatile disks (DVDs) -- also referred to as digital video disks -- and Blu-ray disks, which provide the highest capacities and data transfer rates of the three.

How do optical disks work?

Optical disks rely on a red or blue laser to record and read data. Most of today's optical disks are flat, circular and 12 centimeters in diameter. Data is stored on the disk in the form of microscopic data pits and lands. The pits are etched into a reflective layer of recording material. The lands are the flat, unindented areas surrounding the pits.

The type of material selected for the recording material depends on how the disk is used. Prerecorded disks such as those created for audio and video recordings can use cheaper material like aluminum foil. Write-once disks and rewritable disks require a more expensive layer of material to accommodate other types of digital data storage.

Data is written to an optical disk in a radial pattern starting near the center. An optical disk drive uses a laser beam to read the data from the disk as it is spinning. It distinguishes between the pits and lands based on how the light reflects off the recording material. The drive uses the differences in reflectivity to determine the 0 and 1 bits that represent the data.

Optical disk vs. magnetic storage media

When first introduced for commercial use, the optical disk could hold much more data than similarly sized magnetic storage media, but improvements in hard disk drive (HDD) technology led to HDDs with much greater capacities on a per-centimeter basis than could be achieved with optical disks. At the same time, solid-state memory technologies continued to improve in both capacity and endurance, while prices steadily dropped.

However, optical disks have one big advantage over other types of storage: durability. Optical storage is less likely to degrade over time compared to magnetic tape, HDDs or solid-state drives (SSDs). The data stored on them is relatively impervious to most environmental threats, such as power surges or magnetic disturbances. Not only does this make optical disks well suited for prerecorded audio and video content, but also for backing up and archiving data, including cold storage.

Optical disk storage capacities

In 2016, Sony announced the development of a disk based on Blu-ray technology that would hold 3.3 terabytes (TB) of data. Although Sony has yet to deliver on this promise, storage capacities have continued to increase with each new generation of optical media:

• A CD can store up to up 700 megabytes (MB) of data.
• A single-layer DVD can hold 4.7 gigabytes (GB) of data, and a double-layer disk can hold 8.5 GB.
• A single-layer Blu-ray disk can store 25 to 33.4 GB of data, and a quad-layer Blu-ray disk can store up to 128 GB of data.

All three formats are available in the same size disks: 120 mm (4.7 inches) in diameter and 1.2 mm (0.05 inches) thick. The consistent sizing makes it possible for Blu-ray drives to support DVDs and CDs and for DVD drives to support CDs. The compatibility works in only one direction, however. CD drives can't run DVDs or Blu-rays, and DVD drives can't run Blu-rays.

Optical disk development and history

The first optical disk, developed in the late 1960s by James T. Russell, stored data as micron-wide dots of light and dark. Russell's optical storage system used a powerful backlight to read the dots through a transparent sheet of material on which the dots were encoded.

While Russell is credited with developing the first optical storage, his creation bears little resemblance to later CDs or DVDs. Russell used transparent foil as the medium and then read the data by shining a light through it. Modern optical disks use a laser to read the light being reflected back from the recording medium. In addition, Russell's system didn't spin as the data was read, so it could be any shape, not just a disk.

The modern CD and DVD are based on technology developed in 1969 in the Netherlands by physicist Peter Kramer while working for Philips Research. Kramer developed the method of encoding data on a reflective metallic foil that could be read via a small, low-powered red laser. The laser assembly read the dots and converted the data to an electrical signal, which was then converted to audio or visual output. His work went on to become the basis of all digital optical storage media, although initially it was used only for analog video on the first LaserDisc.

In the 1970s, Philips teamed up with Sony in a joint consortium focused on optical storage. In 1979, they developed the first audio CD, which marked the beginning of digital optical storage for commercial use. However, the technology didn't receive serious recognition until Philips and Sony came out with the first commercial CD player in 1982. Since then, there has been a constant succession of optical disk formats, first in CD formats and followed by a number of DVD formats.

Five years after releasing the CD player, Sony joined forces with Denon to produce the first CD-ROM for storing all types of digital data, not just audio. The CD-ROM could hold approximately 680 MB of data, which later increased to 700 MB. Almost 10 years after that, Sony again teamed up with Philips, as well as Toshiba and Panasonic, to create the DVD, which increased data capacity to 4.7 GB.

It took another 10 years before the next generation of optical storage, the Blu-ray disk, hit the market. Rather than using a red laser, a Blu-ray disk uses a blue laser, which significantly increases capacities and data transfer rates. Boasting storage of up to 25 GB, the Blu-ray was developed by a consortium that was again led by Sony.

Toshiba did not participate this time, as it had developed and tried to market its own format, the HD-DVD. After a short format war, the Blu-ray emerged as the industry standard.

How optical storage disks are made

Optical disks are inexpensive to manufacture. All modern formats use the same basic sandwich of materials structure. A hard plastic substrate forms the base, and then a reflective layer -- typically aluminum foil for mass-produced disks -- is used to encode the digital data. Next, a layer of clear polycarbonate protects the foil and allows the laser beam to pass through to the reflective layer.

Manufacturers can create prerecorded audio and video optical disks in bulk. They can also create software and computer game distribution disks in bulk, although internet streaming has reduced the need for these types of disks.

When producing prerecorded disks in bulk, manufacturers first build a glass master and, from this master, create a negative disk image made from nickel. They then use this nickel image to physically stamp the digital pits into the reflective foil layer. This enables mass production at a scale not possible by individually encoding optical disks with a laser, as happens when a disk is written, or burned, in a computer.

Optical disks that are intended for digital data storage include different materials for the reflective layer, depending on whether the disk is write-once or rewritable. A write-once optical disk includes an organic dye layer between the unwritten reflective foil and the polycarbonate. Rewritable optical disks swap the aluminum foil for an alloy that is a phase-change material so it can be erased and rewritten multiple times.